1. Field of the Invention
The present invention relates to a stage apparatus for moving an object of positioning, two-dimensionally relative to a predetermined base. More particularly, the present invention relates to such a stage apparatus which may be suitably applied to reticle stages in scanning type projection exposure apparatuses used for the photo-lithographic processes to produce various devices such as semiconductor devices and liquid crystal display devices. 2.
2. Related Art
Various types of projection exposure apparatuses are used for the photolithographic processes to produce, for example, semiconductor devices and liquid crystal display devices. In such projection exposure apparatuses, it has come to be desired to transfer a larger and larger pattern onto a wafer (or a glass substrate, etc.) by exposure, without imposing any severer requirements on the projection lens system of the projection exposure apparatus. For this reason, the scanning type projection exposure apparatuses has been noted, which include the slit-scan type and the step-and-scan type of apparatuses. In a scanning type projection exposure system, a reticle (or a photomask, etc.) is illuminated with an exposure light beam, and the reticle and a wafer are moved in synchronism with each other and relative to the projection lens system in order to carry out the scanning projection exposure, so that the image of the patterns on the reticle are transferred onto the shot areas on the wafer in order.
FIG. 6 shows a conventional step-and-scan type projection exposure apparatus. In FIG. 6, an illumination light beam EL is irradiated from an illumination system (not shown) and illuminates patterns of a slit-shaped illumination field 28 on a reticle 12. The image of the patterns on the reticle 12 is projected through a projection lens system 8 onto and within an exposure field 28W on the wafer 5. Here, we define the direction of the optical axis of the projection lens system 8 as the Z-direction, the direction perpendicular to the Z-direction and parallel to the sheet surface of FIG. 6 as the X-direction, and the direction perpendicular to the sheet surface of FIG. 6 as the Y-direction. When an exposure is made using the scanning type projection exposure technique, the reticle 12 is moved relative to the stationary, slit-shaped illumination field 28 toward +Y-direction (or -Y-direction) with a constant velocity V.sub.R, while the wafer 5 is moved in synchronism with the movement of the reticle 12 toward -Y-direction (or +Y-direction) with a constant velocity .beta..multidot.V.sub.R, for scanning projection (where .beta. is the projection magnification ratio of the projection lens system 8).
The apparatus includes a drive system for driving both the reticle 12 and the wafer 5, which comprises a reticle scan stage 10 movably placed on a reticle support 9 so as to be movable in the Y-direction, and a reticle fine adjustment stage 11 placed on the scan stage 10. The reticle 12 is held on the fine adjustment stage 11 by means of, for example, a vacuum chuck. The scan stage 10 is driven by, for example, a linear motor to move relative to the reticle support 9 in the Y-direction. The fine adjustment stage 11 is driven by actuators, which are described in detail below, to make minute and precise displacements relative to the scan stage 10 in the X-direction, in the Y-direction and in the rotational direction (.theta.-direction), so as to control the position of the reticle 12. Moving mirrors 21 are mounted on the fine adjustment stage 11, and interferometers 14 (for the reticle) are mounted on the reticle support 9. The moving mirrors 21 and the interferometers 14 cooperate to continuously monitor the position of the fine adjustment stage 11 in the X-, Y- and .theta.-directions. The position information S1 acquired through the interferometers 14 is supplied to the main control system 22A. The main control system 22A controls the movements of the scan stage 10 and the fine adjustment stage 11 through a reticle stage drive system 22C.
The drive system further comprises a wafer support 1, a wafer Y-axis drive stage 2 placed on the wafer support 1 for linear motion in the Y-direction, a wafer X-axis drive stage 3 placed on the Y-axis drive stage 2 for linear motion in the X-direction, and a Z/.theta.-axes drive stage 4 placed on the X-axis drive stage 3. The wafer 5 is held on the Z/.theta.-axes drive stage 4 by means of, for example, a vacuum chuck. Moving mirrors 7 are mounted on the Z/.theta.-axes drive stage 4, and interferometers 13 (for the wafer) are mounted at portions of the apparatus outside the wafer support 1. The moving mirrors 7 and the interferometers 13 cooperate to continuously monitor the positions of the Z/.theta.-axes drive stage 4 in the X-, Y- and .theta.-directions. The position information acquired through the interferometers 13 is supplied to the main control system 22A as well. The main control system 22A controls the positioning operation of the wafer Y-axis drive stage 2, the wafer X-axis drive stage 3 and the Z/.theta.-axes drive stage 4 through a wafer stage drive system 22B, and further controls the operation of the entire apparatus.
FIG. 7(a) is a plan view of the wafer stage. In FIG. 7(a), the wafer 5 is placed on the Z/.theta.-axes drive stage 4. There are fixedly mounted on the Z/.theta.-axes drive stage 4 a moving mirror 7X for X-direction measurement and a moving mirror 7Y for Y-direction measurement. Two laser beams LWX and LW.sub.OF fall on the moving mirror 7X. The laser beams LWX and LW.sub.OF have their beam axes extending in the X-direction and spaced a predetermined distance IL from each other in the Y-direction. Two laser beams LWY1 and LWY2 from interferometers 13Y1 and 13Y2 for Y-direction measurement, respectively, fall on the moving mirror 7Y. The laser beams LWY1 and LWY2 have their beam axes extending in the Y-direction and spaced a predetermined distance IL from each other in the X-direction. During an exposure step, the coordinate value measured by the interferometer associated with the laser beam LWX is used as the X-coordinate of the Z/.theta.-axes drive stage 4. Also, the averaged value ((Y.sub.W1 +Y.sub.W2)/2) between the coordinate values Y.sub.W1 and Y.sub.W2 measured by the interferometers 13Y1 and 13Y2, respectively, is used as the Y-coordinate of the Z/.theta.-axes drive stage 4. Further, the rotational angle (angular position) of the Z/.theta.-axes drive stage 4 in the rotational direction (.theta.-direction) is derived, for example, from the difference between the coordinate values Y.sub.W1 and Y.sub.W2. These X- and Y-coordinates and the rotational angle are used to control the scanning velocity, position, and rotational angle of the Z/.theta.-axes drive stage 4 in the XY-plane.
FIG. 7(b) is a plan view of the reticle stage. In FIG. 7(b), the fine adjustment stage 11 is placed on the scan stage 10, and the reticle 12 is held on the fine adjustment stage 11 by means of, for example, a vacuum chuck. Further, there are fixedly mounted on the fine adjustment stage 11 one moving mirror 21x for X-direction measurement and two corner cubes (corner reflectors) 21y1 and 21y2 for Y-direction measurement. A laser beam LRH from an interferometer 14H falls on the moving mirror 21x in the X-direction. Laser beams LRJ and LRR from interferometers 14J and 14R for Y-direction measurement, respectively, fall on the corner cubes 21y1 and 21y2, respectively, in the Y-direction.
The laser beams LRJ and LRR, having been reflected by the corner cubes 21y1 and 21y2, are reflected back by mirrors 27 and 26, respectively. Thus, the interferometers 14J and 14R for the reticle utilize the double-path technique, which will avoid any positional displacement of the laser beam irrespective of the rotation of the fine adjustment stage 11. The interferometers 14J and 14R measure Y-coordinate values independently from each other, the Y-coordinate values being represented here by Y.sub.R1 and Y.sub.R2, respectively.
A further corner cube 24 is fixedly mounted on the scan stage 10, along one of the end side thereof in the +Y-direction. In addition to the above mentioned two interferometers 14J and 14R for the reticle, there is provided a third interferometer 23 for the reticle, which irradiates a laser beam LRY. The laser beam LRY is reflected by the corner cube 24 into a mirror 25, and reflected back by the mirror 25 into the corner cube 24 again, and back to the interferometer 23. The interferometer 23 uses the double-path technique, and continuously monitors the Y-coordinate Y.sub.R3 of the scan stage 10. Like the wafer stage described above, the averaged value ((Y.sub.R1 +Y.sub.R2)/2) between the coordinate values Y.sub.R1 and Y.sub.R2 measured by the interferometers 14J and 14R, respectively, is used as the Y-coordinate Y.sub.R of the fine adjustment stage 11. The coordinate value measured by the interferometer 14H, which utilizes the single-path technique, along the measuring line H which is coincident with the axis of the laser beam LRH, is used as the X-coordinate of the fine adjustment stage 11. This coordinate value can be considered as the displacement H of the fine adjustment stage 11 relative to the scan stage 10 along the measuring line H. Please note that the measuring line H and the displacement H are referred to by using the same sign "H".
It is noted in this relation that the interferometer 14H is fixedly mounted on a stationary potion of the apparatus outside the scan stage 10, with the axis of its laser beam LRH passing through the center of the illumination field 28. Thus, when the scan stage 10 is moved in the Y-direction, the laser beam LRH moves relative to the fine adjustment stage 11 in the -Y-direction along the moving mirror 21x. Further, during a scanning projection exposure, the position of the fine adjustment stage 11 in the X-direction is controlled such that the coordinate value measured by the interferometer 14H is kept to be a constant, provided that there is no offset in the X-direction between the reticle stage and the wafer stage.
Moreover, the displacement of the fine adjustment stage 11 relative to the scan stage 10, as measured along the measuring line J which is coincident with the axis of the laser beam LRJ, is referred to as the "displacement J". Also, the displacement of the fine adjustment stage 11 relative to the scan stage 10, as measured along the measuring line R which is coincident with the axis of the laser beam LRR, is referred to as the "displacement R". Then, the displacement J has a value obtained by subtracting the coordinate value Y.sub.R3 from the coordinate value Y.sub.R1, while the displacement R has a value obtained by subtracting the coordinate value Y.sub.R3 from the coordinate value Y.sub.R2. These relationships are expressed as EQU J=Y.sub.R1 -Y.sub.R3, R=Y.sub.R2 -Y.sub.R3.
Further, the difference between the displacements J and R gives the rotational angle .theta. of the fine adjustment stage 11 relative to the scan stage 10.
There are fixedly mounted on the scan stage 10 three actuators 29j, 29r and 29h. The actuators 29j and 29r are spaced a predetermined distance from each other in the X-direction and drive the fine adjustment stage 11 to displace along the driving lines j and r extending in the Y-direction, while the actuator 29h drives the fine adjustment stage 11 to displace along the driving line h extending in the X-direction. The fine adjustment stage 11 may be minutely rotated by controlling the displacements to be produced by the actuators 29j and 29r, respectively. The actuator 29j comprises a rod 31j having its tip end pivotally engaged with a V-shaped groove formed in one of the side edges of the fine adjustment stage 11, and a drive motor 30j for driving the rod 31j to move in its longitudinal direction. The rod 31j has its proximal end connected with the drive motor 30j through a pivotal connection allowing a small tilt of the rod 31j. The other actuators 29r and 29h have the same construction as that described above, respectively. There are also mounted on the scan stage 10 three compression coil springs 32j, 32r and 32h associated with the three actuators 29j, 29r and 29h, respectively, for urging the fine adjustment stage 11 toward the associated actuators. By these springs 32j, 32r and 32h, the fine adjustment stage 11 is normally forced to abut against the rods of the actuators 29j, 29r and 29h. In the actuators 29j, 29r and 29h, a rotational angle of a rotary motor is converted into a linear displacement through a screw rod mechanism.
Here, the projection magnification ratio of the projection lens system 8 is represented by .beta.. When a scanning projection exposure is made, the scan stage 10 in FIG. 7(b) is moved in +Y-direction (or -Y-direction) with a velocity V.sub.R, while the Z/.theta.-axes drive stage 4 in FIG. 7(a) is moved, in synchronism with the movement of the scan stage 10, in -Y-direction (or +Y-direction) with a velocity .beta..multidot.V.sub.R, for scanning. That is, the design value of the velocity ratio between the scan stage 10 and the Z/.theta.-axes drive stage 4 is .beta.. However, the actual velocity ratio may have an error and thus may deviate from the design value of .beta.. Further, it is also possible that the relative rotational angle between the scan stage 10 and the Z/.theta.-axes drive stage 4 may exceed a predetermined allowable value. Under these situations, it is required to control the position and/or the rotational angle of the fine adjustment stage 11 relative to the scan stage 10 (see FIG. 7(b)) in order to null the error in the velocity ratio and/or the error in the relative rotational angle. The fine adjustment stage 11 is much lighter in weight than the scan stage 10 to provide a quicker response, so that the control of the position and/or the rotational angle can be made quickly even during a scanning projection exposure.
FIG. 8 shows a control system for controlling the rotational speed of the rotary motor in the actuator 29h in FIG. 7(b). In FIG. 8, a speed error signal I (in volts) from a displacement-to-voltage conversion unit (not shown) is supplied to a subtracter 33A. A rotational speed signal from a tachogenerator (i.e., a tachometer generator) is supplied to the subtracter 33A as well. At the subtracter 33A, the accumulated signal is subtracted from the signal I to produce a differential signal, which is supplied to the amplifier 34A, whose transfer function is represented by E(s). The amplifier 34A outputs a drive signal which drives the rotary motor 35A, whose transfer function is represented by M(s), to cause the rotary motor 35A to rotate at a rotational speed Y.sub.1 (rad./s). The rotational speed Y.sub.1 of the rotary motor 35A is detected by the tachogenerator 36A, whose transfer function is represented by T(s). Also, the rotational speed Y.sub.1 is integrated by an integrator 37A, comprising a rotating shaft, into a rotational angle Y.sub.2 (rad.), which in turn is converted by a threaded member 38A (which is a threaded portion of the rod 31h in FIG. 7(b)) into a displacement Y(m) along the driving line h. With the pitch of the thread of the threaded member 38A being P(m), one complete rotation (=2.pi. rad.) of the rotational angle Y.sub.2 will be converted into the displacement of P(m).
In FIG. 8, the subtracter 33A, the amplifier 34A, the rotary motor 35A and the tachogenerator 36A form together a rotational speed control loop. By replacing the rotational speed control loop in FIG. 8 with a single block 39A named "rotational speed controller", we obtain FIG. 9. The rotational speed controller comprising the above listed components 33A, 34A, 35A and 36A can operate at a much higher speed than the position control loop comprising the threaded member 38A, the interferometer (not shown), and other components. Therefore, we can consider the ratio between the value of the speed error signal I and the value of the rotational speed Y.sub.1 as derived at the rotational speed controller 39A, to be a rotational speed generation constant G (rad./(volt.multidot.s.), and thereby the control system in FIG. 8 can be replaced with that in FIG. 9.
The control system in FIG. 9 may be used not only for the actuator 29h but also for the other actuators 29g and 29r. FIG. 10 shows a block diagram of a conventional drive system, which comprises the mechanisms of the fine adjustment stage 11 in FIG. 7(b) and three such control systems associated with the actuators 29h, 29g and 29r, respectively.
In FIG. 10, a target displacement HT, to be measured by the interferometer 14H along the measuring line H (see FIG. 7(b)), is supplied from a target value setting unit (not shown) to a first input portion of a subtracter 40A. Also, an actual displacement H, actually measured by the interferometer 14H, is supplied to a second input portion of the subtracter 40A. At the subtracter 40A, the actual displacement H is subtracted from the target displacement HT to produce a deviation, which is supplied to a position controller 41A. At the position controller 41A, the deviation is multiplied by a displacement-to-voltage conversion factor K.sub.H to produce a velocity error signal, which is supplied to a rotational speed controller 39A. The rotational speed, which has been realized by the rotational speed controller 39A, is integrated by the integrator 37A, and the integrated value is converted by the threaded member 38A into a displacement along the driving line h. The resulting total displacement along the driving line h at the tip end of the threaded member 38A is referred to as the "displacement h".
In parallel with the above described sequence of components 40A, 41A, 39A, 37A and 38A, there are provided two similar sequences of components 40B, 41B, 39B, 37B and 38B; 40C, 41C, 39C, 37C and 38C for controlling the actuators 29j and 29r, respectively, in FIG. 7(b). Specifically, the sequence of components including a subtracter 40B, a position controller 41B, a rotational speed controller 39B, an integrator 37B and a threaded member 38B converts a target displacement JT of the fine adjustment stage 11 relative to the scan stage 10, to be measured by the interferometer 14J along the measuring line J which is coincident with the optical axis of the interferometer 14J (see FIG. 7(b)), into the displacement j along the driving line j at the tip end of the threaded member 38B of the actuator 29j. Similarly, the sequence of components including a subtracter 40C, a position controller 41C, a rotational speed controller 39C, an integrator 37C and a threaded member 38C converts a target displacement RT of the fine adjustment stage 11 relative to the scan stage 10, to be measured by the interferometer 14R along the measuring line R which is coincident with the optical axis of the interferometer 14R (see FIG. 7(b)), into the displacement r along the driving line r at the tip end of the threaded member 38C of the actuator 29r.
A mechanical conversion unit 42 in FIG. 10 corresponds to the mechanisms of the fine adjustment stage 11 in FIG. 7(b). When three displacements h, j and r are produced by the actuators 29h, 29j and 29r, respectively, the mechanisms of the fine adjustment stage 11 provides. corresponding set of three displacements H, J and R (of the fine adjustment stage 11 relative to the scan stage 10) detected along the optical axes of the three interferometers 14H, 14J and 14R, respectively. This mechanical operation is represented by a matrix T shown in the box of the mechanical conversion unit 42 in FIG. 10, which is arranged in three rows and three columns (i.e., it is a 3.times.3 matrix).